10.17 Threading
Micky R. Jennings
What is it?
Threading is the process of creating a precision angular helical groove down the length of a shaft. The result of this cutting operation is known as a thread. There are many types of threads that could be discussed, but by far the most common are unified inch threads and metric threads. In this documentation, The basics of single point cutting unified threads. This process can easily be adapted for cutting metric threads. Before beginning to cut threads, it is important to define some thread terminology.
Terminology
Nominal Diameter
The nominal diameter of the thread is how it is generally referred to. For example, a 1/2″ bolt. The nominal diameter is 1/2″, but that isn’t what the outside of the thread really measures.
Major Diameter
The major diameter of an external thread is what the outside diameter of the thread actually measures, generally slightly smaller than nominal. For internal threads, the major diameter is the largest diameter of the thread being cut.
Minor Diameter
The minor diameter of an external thread refers to the smallest diameter of the thread being cut. For internal threads, the minor diameter is the same as the tap drill size when looking to maintain 75% thread engagement.
Pitch Diameter
The pitch diameter is the imaginary diameter that exists at the point where 1/2 of the thread’s pitch would lie horizontally on a single thread. This is one of the most important parts of a thread to measure because it is essentially where internal and external threads meet.
Threads Per Inch
Also known as TPI, threads per inch is used in describing inch thread sizes. For example, a 1/2-13 thread has a nominal diameter of 1/2″ and a TPI of 13. Metric threads don’t use TPI to describe threads; they just use the thread pitch.
Pitch
The pitch of a thread is the distance from one point on a thread to the same point on another thread. Knowing the size of a thread, the pitch can be calculated with the formula:
Pitch = 1/TPI
For example, a 1/2-13 thread has a pitch of .077 because 1/13 = .077
Lead
The lead of a thread is the distance a threaded shaft would travel inside a nut when turned one full rotation. For a single start thread, the lead is equal to the thread pitch. For a double start thread, the lead would be twice the thread pitch, and so on.
Crest
The crest of a thread is the small flat at the major diameter of an external thread and the Minor diameter of an internal thread.
Root
The rootof a thread is the bottom surface generated by the thread cutting tool. On an external thread, the root would be at the minor diameter, and on an internal thread, the root would be at the major diameter.
Flank
The flank of the thread is the angular portion between the crest and root. The vee shape cut on the thread, from one flank to the other flank, measures 60 degrees on both inch and metric threads.
Step by step process for external threading:
- Turn the outside major diameter of the area to be threaded. The m